TitleSession Initiation Protocol (SIP): Locating SIP Servers
AuthorJ. Rosenberg, H. Schulzrinne
DateJune 2002
Format:TXT, HTML
Updated byRFC7984, RFC8553

Network Working Group                                       J. Rosenberg
Request for Comments: 3263                                   dynamicsoft
Obsoletes: 2543                                           H. Schulzrinne
Category: Standards Track                                    Columbia U.
                                                               June 2002

        Session Initiation Protocol (SIP): Locating SIP Servers

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   The Session Initiation Protocol (SIP) uses DNS procedures to allow a
   client to resolve a SIP Uniform Resource Identifier (URI) into the IP
   address, port, and transport protocol of the next hop to contact.  It
   also uses DNS to allow a server to send a response to a backup client
   if the primary client has failed.  This document describes those DNS
   procedures in detail.

Table of Contents

   1          Introduction ........................................    2
   2          Problems DNS is Needed to Solve .....................    2
   3          Terminology .........................................    5
   4          Client Usage ........................................    5
   4.1        Selecting a Transport Protocol ......................    6
   4.2        Determining Port and IP Address .....................    8
   4.3        Details of RFC 2782 Process .........................    9
   4.4        Consideration for Stateless Proxies .................   10
   5          Server Usage ........................................   11
   6          Constructing SIP URIs ...............................   12
   7          Security Considerations .............................   12
   8          The Transport Determination Application .............   13
   9          IANA Considerations .................................   14
   10         Acknowledgements ....................................   14
   11         Normative References ................................   15
   12         Informative References ..............................   15

RFC 3263               SIP: Locating SIP Servers               June 2002

   13         Authors' Addresses ..................................   16
   14         Full Copyright Statement ............................   17

1 Introduction

   The Session Initiation Protocol (SIP) (RFC 3261 [1]) is a client-
   server protocol used for the initiation and management of
   communications sessions between users.  SIP end systems are called
   user agents, and intermediate elements are known as proxy servers.  A
   typical SIP configuration, referred to as the SIP "trapezoid", is
   shown in Figure 1.  In this diagram, a caller in domain A (UA1)
   wishes to call Joe in domain B (joe@B).  To do so, it communicates
   with proxy 1 in its domain (domain A).  Proxy 1 forwards the request
   to the proxy for the domain of the called party (domain B), which is
   proxy 2.  Proxy 2 forwards the call to the called party, UA 2.

   As part of this call flow, proxy 1 needs to determine a SIP server
   for domain B.  To do this, proxy 1 makes use of DNS procedures, using
   both SRV [2] and NAPTR [3] records.  This document describes the
   specific problems that SIP uses DNS to help solve, and provides a

2 Problems DNS is Needed to Solve

   DNS is needed to help solve two aspects of the general call flow
   described in the Introduction.  The first is for proxy 1 to discover
   the SIP server in domain B, in order to forward the call for joe@B.
   The second is for proxy 2 to identify a backup for proxy 1 in the
   event it fails after forwarding the request.

   For the first aspect, proxy 1 specifically needs to determine the IP
   address, port, and transport protocol for the server in domain B.
   The choice of transport protocol is particularly noteworthy.  Unlike
   many other protocols, SIP can run over a variety of transport
   protocols, including TCP, UDP, and SCTP.  SIP can also use TLS.
   Currently, use of TLS is defined for TCP only.  Thus, clients need to
   be able to automatically determine which transport protocols are
   available.  The proxy sending the request has a particular set of
   transport protocols it supports and a preference for using those
   transport protocols.  Proxy 2 has its own set of transport protocols
   it supports, and relative preferences for those transport protocols.
   All proxies must implement both UDP and TCP, along with TLS over TCP,
   so that there is always an intersection of capabilities.  Some form
   of DNS procedures are needed for proxy 1 to discover the available
   transport protocols for SIP services at domain B, and the relative
   preferences of those transport protocols.  Proxy 1 intersects its
   list of supported transport protocols with those of proxy 2 and then
   chooses the protocol preferred by proxy 2.

RFC 3263               SIP: Locating SIP Servers               June 2002

    ............................          ..............................
    .                          .          .                            .
    .                +-------+ .          . +-------+                  .
    .                |       | .          . |       |                  .
    .                | Proxy |------------- | Proxy |                  .
    .                |   1   | .          . |  2    |                  .
    .                |       | .          . |       |                  .
    .              / +-------+ .          . +-------+ \                .
    .             /            .          .            \               .
    .            /             .          .             \              .
    .           /              .          .              \             .
    .          /               .          .               \            .
    .         /                .          .                \           .
    .        /                 .          .                 \          .
    .       /                  .          .                  \         .
    .   +-------+              .          .                +-------+   .
    .   |       |              .          .                |       |   .
    .   |       |              .          .                |       |   .
    .   | UA 1  |              .          .                | UA 2  |   .
    .   |       |              .          .                |       |   .
    .   +-------+              .          .                +-------+   .
    .              Domain A    .          .   Domain B                 .
    ............................          ..............................

                        Figure 1: The SIP trapezoid

   It is important to note that DNS lookups can be used multiple times
   throughout the processing of a call.  In general, an element that
   wishes to send a request (called a client) may need to perform DNS
   processing to determine the IP address, port, and transport protocol
   of a next hop element, called a server (it can be a proxy or a user
   agent).  Such processing could, in principle, occur at every hop
   between elements.

   Since SIP is used for the establishment of interactive communications
   services, the time it takes to complete a transaction between a
   caller and called party is important.  Typically, the time from when
   the caller initiates a call until the time the called party is
   alerted should be no more than a few seconds.  Given that there can
   be multiple hops, each of which is doing DNS lookups in addition to
   other potentially time-intensive operations, the amount of time
   available for DNS lookups at each hop is limited.

   Scalability and high availability are important in SIP. SIP services
   scale up through clustering techniques.  Typically, in a realistic
   version of the network in Figure 1, proxy 2 would be a cluster of
   homogeneously configured proxies.  DNS needs to provide the ability

RFC 3263               SIP: Locating SIP Servers               June 2002

   for domain B to configure a set of servers, along with prioritization
   and weights, in order to provide a crude level of capacity-based load

   SIP assures high availability by having upstream elements detect
   failures.  For example, assume that proxy 2 is implemented as a
   cluster of two proxies, proxy 2.1 and proxy 2.2.  If proxy 1 sends a
   request to proxy 2.1 and the request fails, it retries the request by
   sending it to proxy 2.2.  In many cases, proxy 1 will not know which
   domains it will ultimately communicate with.  That information would
   be known when a user actually makes a call to another user in that
   domain.  Proxy 1 may never communicate with that domain again after
   the call completes.  Proxy 1 may communicate with thousands of
   different domains within a few minutes, and proxy 2 could receive
   requests from thousands of different domains within a few minutes.
   Because of this "many-to-many" relationship, and the possibly long
   intervals between communications between a pair of domains, it is not
   generally possible for an element to maintain dynamic availability
   state for the proxies it will communicate with.  When a proxy gets
   its first call with a particular domain, it will try the servers in
   that domain in some order until it finds one that is available.  The
   identity of the available server would ideally be cached for some
   amount of time in order to reduce call setup delays of subsequent
   calls.  The client cannot query a failed server continuously to
   determine when it becomes available again, since this does not scale.
   Furthermore, the availability state must eventually be flushed in
   order to redistribute load to recovered elements when they come back

   It is possible for elements to fail in the middle of a transaction.
   For example, after proxy 2 forwards the request to UA 2, proxy 1
   fails.  UA 2 sends its response to proxy 2, which tries to forward it
   to proxy 1, which is no longer available.  The second aspect of the
   flow in the introduction for which DNS is needed, is for proxy 2 to
   identify a backup for proxy 1 that it can send the response to.  This
   problem is more realistic in SIP than it is in other transactional
   protocols.  The reason is that some SIP responses can take a long
   time to be generated, because a human user frequently needs to be
   consulted in order to generate that response.  As such, it is not
   uncommon for tens of seconds to elapse between a call request and its

RFC 3263               SIP: Locating SIP Servers               June 2002

3 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [4] and
   indicate requirement levels for compliant SIP implementations.

4 Client Usage

   Usage of DNS differs for clients and for servers.  This section
   discusses client usage.  We assume that the client is stateful
   (either a User Agent Client (UAC) or a stateful proxy).  Stateless
   proxies are discussed in Section 4.4.

   The procedures here are invoked when a client needs to send a request
   to a resource identified by a SIP or SIPS (secure SIP) URI.  This URI
   can identify the desired resource to which the request is targeted
   (in which case, the URI is found in the Request-URI), or it can
   identify an intermediate hop towards that resource (in which case,
   the URI is found in the Route header).  The procedures defined here
   in no way affect this URI (i.e., the URI is not rewritten with the
   result of the DNS lookup), they only result in an IP address, port
   and transport protocol where the request can be sent.  RFC 3261 [1]
   provides guidelines on determining which URI needs to be resolved in
   DNS to determine the host that the request needs to be sent to.  In
   some cases, also documented in [1], the request can be sent to a
   specific intermediate proxy not identified by a SIP URI, but rather,
   by a hostname or numeric IP address.  In that case, a temporary URI,
   used for purposes of this specification, is constructed.  That URI is
   of the form sip:<proxy>, where <proxy> is the FQDN or numeric IP
   address of the next-hop proxy.  As a result, in all cases, the
   problem boils down to resolution of a SIP or SIPS URI in DNS to
   determine the IP address, port, and transport of the host to which
   the request is to be sent.

   The procedures here MUST be done exactly once per transaction, where
   transaction is as defined in [1].  That is, once a SIP server has
   successfully been contacted (success is defined below), all
   retransmissions of the SIP request and the ACK for non-2xx SIP
   responses to INVITE MUST be sent to the same host.  Furthermore, a
   CANCEL for a particular SIP request MUST be sent to the same SIP
   server that the SIP request was delivered to.

   Because the ACK request for 2xx responses to INVITE constitutes a
   different transaction, there is no requirement that it be delivered
   to the same server that received the original request (indeed, if
   that server did not record-route, it will not get the ACK).

RFC 3263               SIP: Locating SIP Servers               June 2002

   We define TARGET as the value of the maddr parameter of the URI, if
   present, otherwise, the host value of the hostport component of the
   URI.  It identifies the domain to be contacted.  A description of the
   SIP and SIPS URIs and a definition of these parameters can be found
   in [1].

   We determine the transport protocol, port and IP address of a
   suitable instance of TARGET in Sections 4.1 and 4.2.

4.1 Selecting a Transport Protocol

   First, the client selects a transport protocol.

   If the URI specifies a transport protocol in the transport parameter,
   that transport protocol SHOULD be used.

   Otherwise, if no transport protocol is specified, but the TARGET is a
   numeric IP address, the client SHOULD use UDP for a SIP URI, and TCP
   for a SIPS URI.  Similarly, if no transport protocol is specified,
   and the TARGET is not numeric, but an explicit port is provided, the
   client SHOULD use UDP for a SIP URI, and TCP for a SIPS URI.  This is
   because UDP is the only mandatory transport in RFC 2543 [6], and thus
   the only one guaranteed to be interoperable for a SIP URI.  It was
   also specified as the default transport in RFC 2543 when no transport
   was present in the SIP URI.  However, another transport, such as TCP,
   MAY be used if the guidelines of SIP mandate it for this particular
   request.  That is the case, for example, for requests that exceed the
   path MTU.

   Otherwise, if no transport protocol or port is specified, and the
   target is not a numeric IP address, the client SHOULD perform a NAPTR
   query for the domain in the URI.  The services relevant for the task
   of transport protocol selection are those with NAPTR service fields
   with values "SIP+D2X" and "SIPS+D2X", where X is a letter that
   corresponds to a transport protocol supported by the domain.  This
   specification defines D2U for UDP, D2T for TCP, and D2S for SCTP.  We
   also establish an IANA registry for NAPTR service name to transport
   protocol mappings.

   These NAPTR records provide a mapping from a domain to the SRV record
   for contacting a server with the specific transport protocol in the
   NAPTR services field.  The resource record will contain an empty
   regular expression and a replacement value, which is the SRV record
   for that particular transport protocol.  If the server supports
   multiple transport protocols, there will be multiple NAPTR records,
   each with a different service value.  As per RFC 2915 [3], the client
   discards any records whose services fields are not applicable.  For
   the purposes of this specification, several rules are defined.

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   First, a client resolving a SIPS URI MUST discard any services that
   do not contain "SIPS" as the protocol in the service field.  The
   converse is not true, however.  A client resolving a SIP URI SHOULD
   retain records with "SIPS" as the protocol, if the client supports
   TLS.  Second, a client MUST discard any service fields that identify
   a resolution service whose value is not "D2X", for values of X that
   indicate transport protocols supported by the client.  The NAPTR
   processing as described in RFC 2915 will result in the discovery of
   the most preferred transport protocol of the server that is supported
   by the client, as well as an SRV record for the server.  It will also
   allow the client to discover if TLS is available and its preference
   for its usage.

   As an example, consider a client that wishes to resolve
   sip:user@example.com.  The client performs a NAPTR query for that
   domain, and the following NAPTR records are returned:

   ;          order pref flags service      regexp  replacement
      IN NAPTR 50   50  "s"  "SIPS+D2T"     ""  _sips._tcp.example.com.
      IN NAPTR 90   50  "s"  "SIP+D2T"      ""  _sip._tcp.example.com
      IN NAPTR 100  50  "s"  "SIP+D2U"      ""  _sip._udp.example.com.

   This indicates that the server supports TLS over TCP, TCP, and UDP,
   in that order of preference.  Since the client supports TCP and UDP,
   TCP will be used, targeted to a host determined by an SRV lookup of
   _sip._tcp.example.com.  That lookup would return:

   ;;          Priority Weight Port   Target
       IN SRV  0        1      5060   server1.example.com
       IN SRV  0        2      5060   server2.example.com

   If a SIP proxy, redirect server, or registrar is to be contacted
   through the lookup of NAPTR records, there MUST be at least three
   records - one with a "SIP+D2T" service field, one with a "SIP+D2U"
   service field, and one with a "SIPS+D2T" service field.  The records
   with SIPS as the protocol in the service field SHOULD be preferred
   (i.e., have a lower value of the order field) above records with SIP
   as the protocol in the service field.  A record with a "SIPS+D2U"
   service field SHOULD NOT be placed into the DNS, since it is not
   possible to use TLS over UDP.

   It is not necessary for the domain suffixes in the NAPTR replacement
   field to match the domain of the original query (i.e., example.com
   above).  However, for backwards compatibility with RFC 2543, a domain
   MUST maintain SRV records for the domain of the original query, even
   if the NAPTR record is in a different domain.  As an example, even
   though the SRV record for TCP is _sip._tcp.school.edu, there MUST
   also be an SRV record at _sip._tcp.example.com.

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      RFC 2543 will look up the SRV records for the domain directly.  If
      these do not exist because the NAPTR replacement points to a
      different domain, the client will fail.

   For NAPTR records with SIPS protocol fields, (if the server is using
   a site certificate), the domain name in the query and the domain name
   in the replacement field MUST both be valid based on the site
   certificate handed out by the server in the TLS exchange.  Similarly,
   the domain name in the SRV query and the domain name in the target in
   the SRV record MUST both be valid based on the same site certificate.
   Otherwise, an attacker could modify the DNS records to contain
   replacement values in a different domain, and the client could not
   validate that this was the desired behavior or the result of an

   If no NAPTR records are found, the client constructs SRV queries for
   those transport protocols it supports, and does a query for each.
   Queries are done using the service identifier "_sip" for SIP URIs and
   "_sips" for SIPS URIs.  A particular transport is supported if the
   query is successful.  The client MAY use any transport protocol it
   desires which is supported by the server.

      This is a change from RFC 2543.  It specified that a client would
      lookup SRV records for all transports it supported, and merge the
      priority values across those records.  Then, it would choose the
      most preferred record.

   If no SRV records are found, the client SHOULD use TCP for a SIPS
   URI, and UDP for a SIP URI.  However, another transport protocol,
   such as TCP, MAY be used if the guidelines of SIP mandate it for this
   particular request.  That is the case, for example, for requests that
   exceed the path MTU.

4.2 Determining Port and IP Address

   Once the transport protocol has been determined, the next step is to
   determine the IP address and port.

   If TARGET is a numeric IP address, the client uses that address.  If
   the URI also contains a port, it uses that port.  If no port is
   specified, it uses the default port for the particular transport

   If the TARGET was not a numeric IP address, but a port is present in
   the URI, the client performs an A or AAAA record lookup of the domain
   name.  The result will be a list of IP addresses, each of which can
   be contacted at the specific port from the URI and transport protocol

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   determined previously.  The client SHOULD try the first record.  If
   an attempt should fail, based on the definition of failure in Section
   4.3, the next SHOULD be tried, and if that should fail, the next
   SHOULD be tried, and so on.

      This is a change from RFC 2543.  Previously, if the port was
      explicit, but with a value of 5060, SRV records were used.  Now, A
      or AAAA records will be used.

   If the TARGET was not a numeric IP address, and no port was present
   in the URI, the client performs an SRV query on the record returned
   from the NAPTR processing of Section 4.1, if such processing was
   performed.  If it was not, because a transport was specified
   explicitly, the client performs an SRV query for that specific
   transport, using the service identifier "_sips" for SIPS URIs.  For a
   SIP URI, if the client wishes to use TLS, it also uses the service
   identifier "_sips" for that specific transport, otherwise, it uses
   "_sip".  If the NAPTR processing was not done because no NAPTR
   records were found, but an SRV query for a supported transport
   protocol was successful, those SRV records are selected. Irregardless
   of how the SRV records were determined, the procedures of RFC 2782,
   as described in the section titled "Usage rules" are followed,
   augmented by the additional procedures of Section 4.3 of this

   If no SRV records were found, the client performs an A or AAAA record
   lookup of the domain name.  The result will be a list of IP
   addresses, each of which can be contacted using the transport
   protocol determined previously, at the default port for that
   transport.  Processing then proceeds as described above for an
   explicit port once the A or AAAA records have been looked up.

4.3 Details of RFC 2782 Process

   RFC 2782 spells out the details of how a set of SRV records are
   sorted and then tried.  However, it only states that the client
   should "try to connect to the (protocol, address, service)" without
   giving any details on what happens in the event of failure.  Those
   details are described here for SIP.

   For SIP requests, failure occurs if the transaction layer reports a
   503 error response or a transport failure of some sort (generally,
   due to fatal ICMP errors in UDP or connection failures in TCP).
   Failure also occurs if the transaction layer times out without ever
   having received any response, provisional or final (i.e., timer B or
   timer F in RFC 3261 [1] fires).  If a failure occurs, the client
   SHOULD create a new request, which is identical to the previous, but

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   has a different value of the Via branch ID than the previous (and
   therefore constitutes a new SIP transaction).  That request is sent
   to the next element in the list as specified by RFC 2782.

4.4 Consideration for Stateless Proxies

   The process of the previous sections is highly stateful.  When a
   server is contacted successfully, all retransmissions of the request
   for the transaction, as well as ACK for a non-2xx final response, and
   CANCEL requests for that transaction, MUST go to the same server.

   The identity of the successfully contacted server is a form of
   transaction state.  This presents a challenge for stateless proxies,
   which still need to meet the requirement for sending all requests in
   the transaction to the same server.

   The problem is similar, but different, to the problem of HTTP
   transactions within a cookie session getting routed to different
   servers based on DNS randomization.  There, such distribution is not
   a problem.  Farms of servers generally have common back-end data
   stores, where the session data is stored.  Whenever a server in the
   farm receives an HTTP request, it takes the session identifier, if
   present, and extracts the needed state to process the request.  A
   request without a session identifier creates a new one.  The problem
   with stateless proxies is at a lower layer; it is retransmitted
   requests within a transaction that are being potentially spread
   across servers.  Since none of these retransmissions carries a
   "session identifier" (a complete dialog identifier in SIP terms), a
   new dialog would be created identically at each server.  This could,
   for example result in multiple phone calls to be made to the same
   phone.  Therefore, it is critical to prevent such a thing from
   happening in the first place.

   The requirement is not difficult to meet in the simple case where
   there were no failures when attempting to contact a server.  Whenever
   the stateless proxy receives the request, it performs the appropriate
   DNS queries as described above.  However, the procedures of RFC 2782
   are not guaranteed to be deterministic.  This is because records that
   contain the same priority have no specified order.  The stateless
   proxy MUST define a deterministic order to the records in that case,
   using any algorithm at its disposal.  One suggestion is to
   alphabetize them, or, more generally, sort them by ASCII-compatible
   encoding.  To make processing easier for stateless proxies, it is
   RECOMMENDED that domain administrators make the weights of SRV
   records with equal priority different (for example, using weights of
   1000 and 1001 if two servers are equivalent, rather than assigning
   both a weight of 1000), and similarly for NAPTR records.  If the
   first server is contacted successfully, the proxy can remain

RFC 3263               SIP: Locating SIP Servers               June 2002

   stateless.  However, if the first server is not contacted
   successfully, and a subsequent server is, the proxy cannot remain
   stateless for this transaction.  If it were stateless, a
   retransmission could very well go to a different server if the failed
   one recovers between retransmissions.  As such, whenever a proxy does
   not successfully contact the first server, it SHOULD act as a
   stateful proxy.

   Unfortunately, it is still possible for a stateless proxy to deliver
   retransmissions to different servers, even if it follows the
   recommendations above.  This can happen if the DNS TTLs expire in the
   middle of a transaction, and the entries had changed.  This is
   unavoidable.  Network implementors should be aware of this
   limitation, and not use stateless proxies that access DNS if this
   error is deemed critical.

5 Server Usage

   RFC 3261 [1] defines procedures for sending responses from a server
   back to the client.  Typically, for unicast UDP requests, the
   response is sent back to the source IP address where the request came
   from, using the port contained in the Via header.  For reliable
   transport protocols, the response is sent over the connection the
   request arrived on.  However, it is important to provide failover
   support when the client element fails between sending the request and
   receiving the response.

   A server, according to RFC 3261 [1], will send a response on the
   connection it arrived on (in the case of reliable transport
   protocols), and for unreliable transport protocols, to the source
   address of the request, and the port in the Via header field.  The
   procedures here are invoked when a server attempts to send to that
   location and that response fails (the specific conditions are
   detailed in RFC 3261). "Fails" is defined as any closure of the
   transport connection the request came in on before the response can
   be sent, or communication of a fatal error from the transport layer.

   In these cases, the server examines the value of the sent-by
   construction in the topmost Via header.  If it contains a numeric IP
   address, the server attempts to send the response to that address,
   using the transport protocol from the Via header, and the port from
   sent-by, if present, else the default for that transport protocol.
   The transport protocol in the Via header can indicate "TLS", which
   refers to TLS over TCP.  When this value is present, the server MUST
   use TLS over TCP to send the response.

RFC 3263               SIP: Locating SIP Servers               June 2002

   If, however, the sent-by field contained a domain name and a port
   number, the server queries for A or AAAA records with that name.  It
   tries to send the response to each element on the resulting list of
   IP addresses, using the port from the Via, and the transport protocol
   from the Via (again, a value of TLS refers to TLS over TCP).  As in
   the client processing, the next entry in the list is tried if the one
   before it results in a failure.

   If, however, the sent-by field contained a domain name and no port,
   the server queries for SRV records at that domain name using the
   service identifier "_sips" if the Via transport is "TLS", "_sip"
   otherwise, and the transport from the topmost Via header ("TLS"
   implies that the transport protocol in the SRV query is TCP).  The
   resulting list is sorted as described in [2], and the response is
   sent to the topmost element on the new list described there.  If that
   results in a failure, the next entry on the list is tried.

6 Constructing SIP URIs

   In many cases, an element needs to construct a SIP URI for inclusion
   in a Contact header in a REGISTER, or in a Record-Route header in an
   INVITE.  According to RFC 3261 [1], these URIs have to have the
   property that they resolve to the specific element that inserted
   them.  However, if they are constructed with just an IP address, for


   then should the element fail, there is no way to route the request or
   response through a backup.

   SRV provides a way to fix this.  Instead of using an IP address, a
   domain name that resolves to an SRV record can be used:


   The SRV records for a particular target can be set up so that there
   is a single record with a low value for the priority field
   (indicating the preferred choice), and this record points to the
   specific element that constructed the URI.  However, there are
   additional records with higher values of the priority field that
   point to backup elements that would be used in the event of failure.
   This allows the constraint of RFC 3261 [1] to be met while allowing
   for robust operation.

RFC 3263               SIP: Locating SIP Servers               June 2002

7 Security Considerations

   DNS NAPTR records are used to allow a client to discover that the
   server supports TLS.  An attacker could potentially modify these
   records, resulting in a client using a non-secure transport when TLS
   is in fact available and preferred.

   This is partially mitigated by the presence of the sips URI scheme,
   which is always sent only over TLS.  An attacker cannot force a bid
   down through deletion or modification of DNS records.  In the worst
   case, they can prevent communication from occurring by deleting all
   records.  A sips URI itself is generally exchanged within a secure
   context, frequently on a business card or secure web page, or within
   a SIP message which has already been secured with TLS.  See RFC 3261
   [1] for details.  The sips URI is therefore preferred when security
   is truly needed, but we allow TLS to be used for requests resolved by
   a SIP URI to allow security that is better than no TLS at all.

   The bid down attack can also be mitigated through caching.  A client
   which frequently contacts the same domain SHOULD cache whether or not
   its NAPTR records contain SIPS in the services field.  If such
   records were present, but in later queries cease to appear, it is a
   sign of a potential attack.  In this case, the client SHOULD generate
   some kind of alert or alarm, and MAY reject the request.

   An additional problem is that proxies, which are intermediaries
   between the users of the system, are frequently the clients that
   perform the NAPTR queries.  It is therefore possible for a proxy to
   ignore SIPS entries even though they are present, resulting in
   downgraded security.  There is very little that can be done to
   prevent such attacks.  Clients are simply dependent on proxy servers
   for call completion, and must trust that they implement the protocol
   properly in order for security to be provided.  Falsifying DNS
   records can be done by tampering with wire traffic (in the absence of
   DNSSEC), whereas compromising and commandeering a proxy server
   requires a break-in, and is seen as the considerably less likely
   downgrade threat.

8 The Transport Determination Application

   This section more formally defines the NAPTR usage of this
   specification, using the Dynamic Delegation Discovery System (DDDS)
   framework as a guide [7].  DDDS represents the evolution of the NAPTR
   resource record.  DDDS defines applications, which can make use of
   the NAPTR record for specific resolution services.  This application
   is called the Transport Determination Application, and its goal is to
   map an incoming SIP or SIPS URI to a set of SRV records for the
   various servers that can handle the URI.

RFC 3263               SIP: Locating SIP Servers               June 2002

   The following is the information that DDDS requests an application to

      Application Unique String: The Application Unique String (AUS) is
         the input to the resolution service.  For this application, it
         is the URI to resolve.

      First Well Known Rule: The first well known rule extracts a key
         from the AUS.  For this application, the first well known rule
         extracts the host portion of the SIP or SIPS URI.

      Valid Databases: The key resulting from the first well known rule
         is looked up in a single database, the DNS [8].

      Expected Output: The result of the application is an SRV record
         for the server to contact.

9 IANA Considerations

   The usage of NAPTR records described here requires well known values
   for the service fields for each transport supported by SIP.  The
   table of mappings from service field values to transport protocols is
   to be maintained by IANA.  New entries in the table MAY be added
   through the publication of standards track RFCs, as described in RFC
   2434 [5].

   The registration in the RFC MUST include the following information:

      Service Field: The service field being registered.  An example for
         a new fictitious transport protocol called NCTP might be

      Protocol: The specific transport protocol associated with that
         service field.  This MUST include the name and acronym for the
         protocol, along with reference to a document that describes the
         transport protocol.  For example - "New Connectionless
         Transport Protocol (NCTP), RFC 5766".

      Name and Contact Information: The name, address, email address and
         telephone number for the person performing the registration.

   The following values have been placed into the registry:

   Services Field               Protocol
   SIP+D2T                       TCP
   SIPS+D2T                      TCP
   SIP+D2U                       UDP
   SIP+D2S                       SCTP (RFC 2960)

RFC 3263               SIP: Locating SIP Servers               June 2002

10 Acknowledgements

   The authors would like to thank Randy Bush, Leslie Daigle, Patrik
   Faltstrom, Jo Hornsby, Rohan Mahy, Allison Mankin, Michael Mealling,
   Thomas Narten, and Jon Peterson for their useful comments.

11 Normative References

   [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [2]   Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
         Specifying the Location of Services (DNS SRV)", RFC 2782,
         February 2000.

   [3]   Mealling, M. and R. Daniel, "The Naming Authority Pointer
         (NAPTR) DNS Resource Record", RFC 2915, September 2000.

   [4]   Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [5]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434, October

12 Informative References

   [6]   Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
         "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [7]   Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         One: The Comprehensive DDDS Standard", Work in Progress.

   [8]   Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         Three: The DNS Database", Work in Progress.

RFC 3263               SIP: Locating SIP Servers               June 2002

13 Authors' Addresses

   Jonathan Rosenberg
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936

   EMail: jdrosen@dynamicsoft.com

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY 10027-7003

   EMail: schulzrinne@cs.columbia.edu

RFC 3263               SIP: Locating SIP Servers               June 2002

14  Full Copyright Statement

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